How do I visualize bounding boxes in different colors?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hello,

I'm trying to visualize point clouds and 3D object detection bounding boxes, on a KITTI-format dataset. I'd like to show bounding boxes in different colors, e.g. green for ground truth and red for predictions.

Following the readme and the visualizer example in scripts/demo_obj_det.py, I'm able to run PointPillars inference and show the point cloud + bounding boxes in the visualizer, but I'm not sure how to specify different colors for bounding boxes. Based on #392 it seems like this should be possible?

I am using Open3D 0.14.1, installed with conda in Python 3.8, and the Open3D-ML installation bundled with this.

Thank you for your help with this!